Arachnida in which the
prosoma is uniform and bears two median and six or ten lateral eyes and
the opisthosoma is divisible into mesosoma and metasoma, each of six
somites.
Pedicle absent; telson in the form of a poison-sting. Chelicerae of three
segments, small, chelate. Pedipalpi of six segments, large, powerful,
chelate. Legs of seven segments, tarsi with three claws. Sternum
triangular or pentagonal. Second mesosomatic somite bears a pair of
characteristic pectines. Respiration by four pairs of lung-books on third
to sixth opisthosomatic somites.